Synchronizing digital audio to digital video

ABSTRACT

An audio/video input/output (I/O) port apparatus for acquiring digital audio samples from one or multiple channels of input audio and synthesizing digital audio samples into one or multiple channels of output audio. The apparatus comprises a video I/O port, a frequency synthesizer, and an audio I/O port. The video I/O port generates a video-rate clock, and is configured to digitize input video into digital video, and to synthesize output video from digital video. The frequency synthesizer is configured to derive an audio sampling clock based on the video-rate clock. The audio I/O port is configured to sample input audio and convert it into digital audio samples according to the sampling clock, and to synthesize digital audio samples into output audio according to the sampling clock. The apparatus ensures that the video and audio data track together, both when inputting the information from an external source and when outputting the audio/video data streams. The technique is particularly valuable in video editing, where it is critical to establish and maintain synchronization between the video of a speaking person and the audio representing the spoken material.

CROSS REFERENCE TO RELATED APPLICATIONS

Patent applications entitled "Displaying a Subsampled Video Image on aComputer Display," "Adaptive Video Compression," "Adaptive VideoDecompression," and "Video Peripheral for a Computer," filed herewith,are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The invention relates to digital processing of video and audio data, andmore particularly to an audio/video input/output port apparatus thatmaintains synchronization between video and audio channels.

As the costs of high-resolution color computer displays and processingpower come down, one of the emerging applications for microcomputers isvideo post production--displaying and editing video images using thedisplay of the computer as the monitor during the editing process. In acomputer video editing system, a video/audio source, typically a videotape recorder, is read, and the data are stored in digital form on thedisk of a computer. The video/audio data may be played back, edited, andwritten back to a video device.

Prior methods for digital processing of video/audio data have allowedthe audio portion of the program to drift from the video portion,resulting in the loss of synchronization.

SUMMARY OF THE INVENTION

The invention provides an apparatus that synchronizes two or morestreams of data, one stream representing video information and theother(s) representing audio information. The apparatus ensures that thevideo and audio data track together, both when inputting the informationfrom an external source and when outputting the separate or joinedaudio/video data streams. The technique is particularly valuable invideo editing, where it is critical to establish and maintainsynchronization between the video of a speaking person and the audiorepresenting the spoken material.

In general, in one aspect, the invention features an audio/videoinput/output (I/O) port apparatus for acquiring digital audio samplesfrom one or multiple channels of input audio and synthesizing digitalaudio samples into one or multiple channels of output audio. Theapparatus comprises a video I/O port, a frequency synthesizer, and anaudio I/O port. The video I/O port generates a video-rate clock, and isconfigured to digitize input analog video into digital video, and tosynthesize output analog video from digital video. The frequencysynthesizer is configured to derive an audio sampling clock based on thevideo-rate clock. The audio I/O port is configured to sample input audiointo digital audio samples according to the sampling clock, and tosynthesize digital audio samples into output audio according to thesampling clock.

In a second aspect, the invention features an audio/video input portapparatus for acquiring one or multiple channels of digital audiosamples. The apparatus comprises a video input port, a frequencysynthesizer, and an audio input port. The video input port is configuredto digitize input video into digital video and generate a video-rateclock. The frequency synthesizer is configured to derive an audiosampling clock based on the video-rate clock. The audio input port isconfigured to sample input audio into digital audio samples according tothe sampling clock.

In a third aspect, the invention features an audio/video output portapparatus for synthesizing digital audio samples into outputaudio/video. The apparatus comprises a video output port, a frequencysynthesizer, and an audio output port. The video output port isconfigured to synthesize output video from digital video and generate avideo-rate clock. The frequency synthesizer is configured to derive anaudio sampling clock based on the video-rate clock. The audio outputport is configured to synthesize output audio from digital audio samplesaccording to the sampling clock.

Preferred embodiments of the invention may include the followingfeatures. The audio/video I/O port apparatus may be a video front end toa video peripheral board mounted in a peripheral slot of a generalpurpose host computer. There may be first-in-first-out buffers (FIFOs)to couple the asynchronous operation of the computer to the synchronousoperation of the video and audio input and output ports. The peripheralboard may have a CODEC for compressing and decompressing the video dataas they are stored and retrieved on a storage device of the hostcomputer. The video input port may also include a detector configured todetect the synchronizing signals of the input video, a frequencygenerator for generating the video-rate clock and clocking the videooutput port, and a pulse generator configured to provide an inputsynchronization pulse generated for each frame of the input video andbased on the detected synchronizing signal, and a pulse generatorconfigured to provide a synchronizing pulse to the audio output port.The video I/O port may generate the video-rate clock independent of bothinput and output video. The frequency synthesizer may include an m/nphase lock loop to generate the audio sampling clock as a ratio of thevideo-rate clock. The electronic implementation of the frequencysynthesizer may allow the ratio of the audio sampling clock to thevideo-rate clock to be varied rapidly depending on the timing of thevideo-rate clock. The frequency synthesizer may also include anoversampled clock synthesizer that generates an oversampled clock whosefrequency is at least twice as great as the frequency of the audiosampling clock, and a frequency divider that frequency divides theoversampled clock to generate the audio sampling clock. The audio/videoI/O port apparatus may further comprise a digital signal processor (DSP)programmed to selectively associate a varying number of digital audiosamples to each respective frame of the digital video in order toachieve a precise ratio of the number of digital audio samples to thenumber of associated video frames. The audio/video I/O port apparatusincludes an analog-to-digital converter and/or a digital-to-analogconverter, the converters being clocked by the audio sampling clock. TheDSP controls the flow between the FIFOs.

Among the advantages of a digital video processor incorporating to theinvention are the following. When audio and video tracks are recordedand later played back, the audio and video will be properlysynchronized--the audio will not run ahead of the video nor vice versa.The audio tracks of a video may be stored separately from the videotrack; this facilitates editing of both the audio and the video tracks.The technique is independent of the incoming video and audio signalformats; i.e., the technique works for different video formats (e.g.NTSC and PAL) and for different audio sampling and/or playback rates.

Other advantages and features of the invention will be apparent from thefollowing description of a preferred embodiment thereof and from theclaims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiment will now be described.

Drawings

FIG. 1 is a diagrammatic perspective view of components used in a videoediting system operating according to the invention.

FIG. 2 is a block diagram of a host computer and a video peripheralboard used in the system of FIG. 1.

FIG. 3 is a block diagram of an alternate organization of the hostcomputer.

FIG. 4 is a block diagram detailing the components of the audio channelof the video editing system and the components that contribute tosynchronizing audio to video.

FIG. 5 is a block diagram of an m/n phase lock loop of the FIG. 4components.

FIG. 6 is a diagram showing the files that store the video and audiotracks of a video clip.

Overview

Referring to FIGS. 1 and 2, video editing system 11 includes peripheralboard 10 that plugs into host computer 12. Other components includevideo tape recorder (VTR) 16, monitor 18, keyboard 20, mouse 22, andmass storage disk 24. The software providing video editing functionalityis divided into two portions, one portion 26 that executes on the hostcomputer's central processing unit (CPU) 28, generally providing a userinterface and supervision, and one portion 14 that executes on theperipheral board, generally controlling the peripheral board, datatransfer within the peripheral board, and data transfer between the hostcomputer and the peripheral.

In video editing system 11, video is read through video input port 30,and audio is read through audio input port 32. As they are read, thevideo is digitized and compressed, and the audio is digitized. The videoand audio are stored on disk 24. The compressed video/audio data may bedecompressed and played back onto display 18 and speakers (not shown).Video editing software 26 allows a user to assemble portions of thecompressed video and audio into a video/audio program. As the user editsthe program, he can play it and rearrange it in small increments, assmall as a single frame, or in assembled combination. Once the user issatisfied with the resulting program, it can be output at full framerates through video output port 34 and audio output port 36 to a videocapture device, e.g. VTR 16, or to a broadcast device.

Referring to FIG. 2, the peripheral board has video and audio ports30-36 (to connect VTR 16 or other video device), bus control circuit 42(to interface with host computer 12), various signal processing paths,and supervisory microprocessor 48. The paths include a two-way paththrough a compression/decompression coder/decoder (CODEC) 60 to transferdigitized video to or from host computer disk 24, and a one-waycolor-space conversion (CSC) and subsample path to display digitizedvideo on host computer display 18. Video input/output (I/O) circuit 35converts the video data from the VTR's analog form, e.g. NTSC or PAL, toa digital form, e.g., YUV 4:2:2 format, and puts the digital video onvideo bus 38. (Video bus 38 can also be written by CODEC 60 during videodecompression.) A microprocessor 48 controls the components of theperipheral board.

During inputting of video, CODEC 60 takes the YUV format video fromvideo bus 38, compresses it into a compressed form, and stores thecompressed video into compression FIFO 92. Bus control circuit 42 takesthe compressed video data from compression FIFO 92 and stores them intobuffers in the host's RAM 50. Host CPU 28 periodically flushes thebuffers to disk 24. Simultaneously, an audio channel transfers data fromVTR 16 (or other audio source) to the peripheral's data bus 46 throughan audio input port 32 and controller 62.

During playback, the process is reversed: host CPU 28 reads thecompressed video data from disk 24 into buffers in RAM 50. Bus controlcircuit 42 copies the data from the buffers to decompression FIFO 94.CODEC 60 drains the decompression FIFO 94, decompresses the data, andoutputs them to video data bus 38. From there, the decompressed videodata can be displayed to the host's display 18 through the subsamplepath, and/or output through video output port 34. Simultaneously, theaudio channel transfers data from the peripheral's data bus 40 throughaudio controller 62 and audio output port 36 to the VTR.

In an alternate organization of the host computer shown in FIG. 3, hostcomputer CPU 28, display memory (also called a "frame buffer") 44, mainmemory 50, and/or disk control 52 components may transfer data through abus 54 private to host computer 12, with bus control interface 56between the private bus 54 and system bus 46. In this case, theperipheral's bus control circuit 42 transfers data to/from the systembus 46, and the host's bus interface 56 further directs the data to/fromthe devices on the host computer's private bus 54.

In any video signal, there is a vertical blanking period to reset theretrace from the bottom of the screen to the top of the screen. Duringthe vertical blanking period, the electron beam is stopped so that itwill not overwrite the previous field. During the vertical blankingperiod, there occurs a synchronization pulse called the "vertical synch"pulse. In interlaced formats (having two interlaced fields per frame),there are two vertical blanking periods per frame, and thus two verticalsynch pulses, one at the end of each field. The two vertical synchpulses can be distinguished by their timing relationships to the actualimage data of the two fields of a frame.

Structure

The apparatus of the invention implements a clocking scheme forrecording and/or playing back one or more audio tracks, especially whenthat audio is synchronized to a video track. The apparatus uses thevideo-rate clock pulses embedded in the video to drive a frequencyconverter that, in turn, generates an audio sampling clock, e.g., at44.1 KHz. The audio sampling clock is used to drive an audio port.

Video I/O port circuit 35 extracts two clocks from the input video data:the pixel-rate line-locked clock 140, either 12.27 MHz for NTSC or 14.75MHz for PAL, and a once-per-frame synch pulse 142, 29.97 Hz for NTSC or25 Hz for PAL. On output, video I/O port circuit 35 synthesizes thevideo clocks to drive the output. Video I/O port circuit 35 is alsoconfigured to synthesize the video-rate clocks, both pixel-rate clock140 and synch pulse 142, even if no actual video is being input oroutput, so that the video clocks can be input to frequency synthesizingcomponents used in inputting and outputting audio, as discussed indetail below. In the latter mode of operation, the invention can be usedto input or output audio independent of any video operation, forinstance to input a new audio track to replace the audio track withwhich video data were originally recorded. Video I/O port circuit 35 mayinput or output composite or S-video.

Pixel-rate clock 140 is fed to programmable m/n phase lock loop 144,described further below, in connection with FIG. 5. Phase lock loop 144generates an oversampled audio clock 146, for instance at 256 times theaudio sampling rate. The oversampled clock is then frequency divideddown by countdown divider 148 to audio sampling clock 150 of, e.g., 44.1KHz. M/n phase lock loop 144 and countdown divider 148 thus togethermake up a frequency synthesizer that derives an audio sampling clockbased on a video-rate clock, namely line-locked pixel rate clock 140.Audio sampling clock 150 is then used to clock sampling of the audioinput at analog-to-digital converter (ADC) 152 or synthesis of the audiooutput at digital-to-analog converter (DAC) 154. Deriving audio samplingclock 150 from the video clock ensures that the audio data are directlysynchronized to the video data on a sample-by-sample basis.

The audio data should also be synchronized to the video data on aframe-by-frame basis, since there may not be an integer number of audiosamples for each frame of video. To handle this, synch pulse 142 of thevideo clock is provided from video I/O port circuit 35 to digital signalprocessor (DSP) 160 as a frame interrupt. The interrupt service routinein DSP 160 adjusts the samples to the video frame rate as will bedescribed below. DSP 160 may be, preferentially, a Texas InstrumentsTMS320C31. In addition to synchronizing audio samples to video frames oninput, DSP 160 also controls the flow of data from/to the ADC FIFO 176and DAC FIFO 186, as will be discussed below, and does a real time mixof multiple audio channels to create two output channels.

Since the rate of video clock 140 differs depending on whether the videois in NTSC or PAL format, the ratio between the pixel rate line-lockedclock 140 and the derived oversampling clock 146 is adjustable to yielda fixed audio sampling rate 150. This is achieved by m/n phase lock loop144, which is shown in FIG. 5 and provides this capability by allowingthe values of m and n to be programmed through the use of counters 310and 312. Thus, the ratio of pixel-rate clock 140 to oversampling clock146 can be adjusted to any rational number. For NTSC and a 44.1 KHzaudio sampling clock, the m/n values are 2423/2634, for PAL they are2287/2988. A 48 KHz sampling clock can be achieved by NTSC m/n values of2413/2410 and PAL values of 2800/3361. A divide-by-n frequencysynthesizer is disclosed in U.S. Pat. No. 5,168,247.

Referring to Fir. 5, m/n phase lock loop 144 includes Motorola MC145145PLL/Synthesizer 300, op amp 302, and voltage controlled oscillator (VCO)304, and components 301, 303. PLL/synthesizer 300 incorporates twocounters 310, 312 and phase comparator 314, and a 4-bit bus interface.Counter 312 counts down from m at the reference frequency, the pixelrate line-locked clock 140 input from video circuit 35. Counter 310counts down from n at the output audio sampling clock 146 rate. Thevalues of m and n can be programmably entered. The outputs ofPLL/synthesizer 300 are phase signals, which are fed to op amp 302. Opamp 302 produces a trimmer signal 306 to tune the output of VCO 304 tothe desired frequency, producing oversampling clock 146.

The values of m and n can be adjusted (programmed) to accommodate anyvideo rate and still produce the desired sampling clock (e.g., 44.1KHz). PLL 144 quickly adjusts to changes in the video clock. This isimportant because video tapes do not always yield fixed rate videoclocking: however the circuit keeps the proper number of samples perframe regardless of the timing (clocking) of any specific frame.

Referring again to FIG. 4, the following discussion presents thestructure and operation for each channel; typically the system willrecord and reproduce stereo, so the circuitry is duplicated for a secondaudio channel.

Each audio input port 32 includes amplifier 172, ADC 152,serial-to-parallel converter 174, and ADC FIFO 176. Each audio outputport 36 comprises DAC FIFO 186, parallel-to-serial converter 184, DAC154, and amplifier 182. FIFOs 176 and 186 are 32 bits wide (for twochannels, each 16 bits wide), 256K deep, using 20 megasample/secondparts. ADC 152 and DAC 154 are 16 bits wide, but the digital data arecommunicated bit serially. Serial-to-parallel converter 174 andparallel-to-serial converter 184 convert the bit-serial stream to the16-bit parallel stream and vice-versa.

Referring to FIG. 6, the three tracks of a program read at video inputport 30 and audio input port 32 are stored on host's disk 24 in fourfiles. The first file 200 stores the compressed video images per se;each frame is identified by a time stamp. The second and third files202, 204 hold the sampled audio for the left and right audio channels,respectively. The audio samples for each channel are stored in thesingle-channel SoundDesigner II format files, in which each audio sampleis stored as 16 bits. The samples are simply ordered by occurrence inthe file and identified by an integer sample number. The fourth type offile 206 contains clip descriptors, each clip descriptor pointing toportions of the video and audio files to be used in the edited finalprogram. Each clip descriptor points into combinations of video filesand one or more of the audio files containing the actual video and audioinformation. As the user edits the program, the actual compressed videoimage or audio sample data need not be copied from disk file to diskfile; the editing is performed by manipulating the pointers in the clipdescriptor file. Then during playback (or recording onto a video device,e.g. a VTR), the clip descriptor files are read in order. For each clip,the indicated video frames and audio samples are read from the disk andpresented to the video and audio output ports, and synchronized asdiscussed below.

Operation

Referring again to FIG. 4, video I/O circuit 35 generates line-lockedclock 140 at the pixel rate and synch pulse 142 at the frame rate.Line-locked pixel rate clock 140 may either be based on video read intovideo I/O port circuit 35 or may be synthesized by video I/O portcircuit 35 independent of any video so that audio can be recorded orplayed back in the absence of video. Depending on whether the video isNTSC or PAL format, the pixel rate, and thus the frequency ofline-locked pixel rate clock 140, varies. The values of m and n arechosen so that m/n phase-lock loop 144 accurately scales line-lockedpixel rate clock 140 to produce oversampled audio clock 146, forinstance 256 times the audio sampling rate of 44.1 KHz. Frequencydivider 148, for instance a countdown-by-256 circuit, produces audiosampling frequency 150, e.g., 44.1 KHz. The audio sampling frequency canbe used either for recording audio input or reproducing audio output.

Audio input, with or without video input, proceeds as follows. Input ADC152 accepts analog audio input from an external audio device throughaudio input port 32, and samples the audio at the rate determined by theaudio sampling frequency. The samples are presented in bit-serial formfrom ADC 152; this is converted to 16-bit parallel form byserial-to-parallel converter 174, and then reformatted into 32-bit wordsand queued in ADC FIFO 176. Because the data are queued in ADC FIFO 176,DSP 160 can empty ADC FIFO 176 at its leisure and need not constantlymonitor an input line to pick up each bit of the input serial stream.

Periodically, e.g., once every 25th or 29.97th of a second on,respectively, a PAL or NTSC frame boundary, video I/O circuit 35 outputsframe synch signal 142. Frame synch 142 is connected to an interruptline of DSP 160. DSP 160 may service the interrupt by adjusting thesamples just before and/or just after the frame boundary to line theaudio samples up with the video frame. For PAL video, whose 25 framesper second is an exact submultiple of the audio sample rate of 44,100samples per second, no adjustment is needed, and each frame has 1,764audio samples associated with it. For NTSC video, with a frame rate of29.97 frames per second, which is not an even submultiple of the audiosampling rate, the system allocates the audio samples to frames tomaintain close synchronization between the audio and the video. DSP 160alternates the number of samples allocated per frame between 1471 and1472. In this way, the audio and video information closely track towithin 1.71 samples in 200 seconds, or 17.1 samples in 33 minutes.

To provide verification of the audio being input to the system, theinput audio data stream is fed back to audio output circuit 36, via theline connecting the output of serial-to-parallel converter 174 to theinput of parallel-to-serial converter 184, so that the user can monitorthe audio data as it is recorded by the video/audio system.

From DSP 160, the sampled audio is placed on DSP data bus 188. Fromthere, it is stored in audio input FIFO 96 before bus control circuit 42takes the audio data and stores them into disk buffers in RAM 50 of thehost computer. Host CPU 28 then flushes the disk buffers to disk 24.

Generally, simultaneously with digitizing and storage of audio, video isdigitized, compressed by CODEC 60 and stored on disk 24. The data paths(video and two audio) through the peripheral and host may treat thethree streams as independent processes. No additional processing isrequired during recording to produce correlation markers that annotatethe relationship of specific frames with specific audio samples. No diskdata structures are required to record the association, except to pointto the first datum of each stream, until the associations are disturbedor reordered during video editing.

Audio output, with or without video output, proceeds as follows. Thethree streams may again be treated as independent, without explicitsynchronization handshaking between them. Because the audio outputsampling clock is driven by the video clock, all that is required isthat the correct number of frames or samples be presented to the videoand audio output ports 34, 36. The synchronized clocks will ensure thatthe output is presented to the analog video device at preciselysynchronized rates.

During playback, video I/O circuit 35 generates a very accurate outputclock that establishes the proper timing of the video played back,regardless of timing variations that may have occurred during recording.(Recorded audio may be played back without any associated video; videoI/O circuit 35 can synthesize video clocks 140, 142 even in the absenceof any actual video data.) Host CPU 28, the peripheral's microprocessor48, and bus control circuit 42 cooperate to move video and audio datafrom the host's disk 24 to decompression FIFO 94 and audio output FIFO98 at rates that ensure that the FIFOs remain non-empty. DSP 160, inturn, drains the audio data from the audio output FIFO 98 into DAC FIFO186.

As video/audio output starts, sampling clock 150 is blocked from audiooutput DAC 154 until the first frame of video is decoded by CODEC 60 andready to be output on video output port 34. Thus, while waiting forvideo output port 34 to start, the audio data are queued in DAC FIFO 186awaiting a synchronization signal from the video output circuit 35 andfor audio sampling clock 150 to begin. As the first frame of video ispresented to video output port 34, audio sampling clock 150 isunblocked, and DSP 160 enables parallel-to-serial converter 184 to drainDAC FIFO 186 into DAC 154.

Once audio output is under way, no further synchronization between thethree data paths is required, only the clock synchronization. Becauseaudio sampling clock 150 is derived from video clock 140, 142, the rateat which audio samples are output at audio output port 32 is forced toremain synchronized with the rate at which video frames are output atvideo output port 34. Since DAC 154 converts the sampled audio at therate dictated by sampling clock 150, and the audio was synchronized tothe video during recording, synchronization between the output video 34and audio 36 is maintained during playback by the sampling clock 150derived from the pixel rate line-lock clock 140 and by the data itself.

During playback, DSP 160 requests data from audio output FIFO 98 atroughly the same time that CODEC 60 is requesting the associated videodata from decompression FIFO 94. As each frame of video is decompressedthrough CODEC 60 and played out through video output port 34, video I/Ocircuit 35 generates a frame interrupt that tells the audio circuit toacquire the next block of audio samples and load them into the audiooutput stream. DSP 160 uses synch 142 to synchronize the presentation tothe DAC 154 of the audio for the corresponding video frame.

Since the audio was synchronized with any incoming video duringrecording, there are the targeted number of audio samples (e.g., 44,100)for each of two stereo audio channels for each second of video. Thetracking of the audio sample rate to the video rate ensures that duringany number of video frames, there will be exactly ##EQU1## samplesrecorded or played back.

Other embodiments of the invention are within the scope of the claims.

This scheme for synchronizing digital audio to digital video is directlyapplicable to other forms of digital video/audio, e.g., High DefinitionTV. Video framing signals (vertical, horizontal, and pixel) will alwaysbe present in some form, and as such will be available as sources forderiving ratioed clock(s) to synchronize previously or simultaneouslyrecorded audio information to that video information.

Multiple format video requires translation to single or limited-formataudio to maintain audio record and regeneration standards in thepresence of different video framing rates. This invention both allowsand facilitates the use of multiple video formats with audio separatelyor simultaneously recorded, and for synchronizing audio to multipleformat video on playback. Even though the audio may be initiallypresented to such a system in digital format, the invention willmaintain synchronization of the audio with independently recorded videoduring playback.

What is claimed is:
 1. An audio/video input/output (I/O) apparatus foracquiring digital audio samples from one or multiple channels of inputaudio and, alternately, synthesizing digital audio samples into one ormultiple channels of output audio, the apparatus comprising:a video I/Oport circuit generating a video-rate clock, said video I/O port circuitcomprising a video input port configured to convert input analog videointo digital video, and a video output port configured to synthesizeoutput analog video from digital video; and an audio I/O port circuitcomprising a frequency synthesizer connected to receive said video-rateclock from said video I/O port and configured to derive am audiosampling clock based on said video-rate clock, an audio input portconfigured to sample and convert input analog audio into digital audiosamples according to said sampling clock, and an audio output portconfigured to synthesize digital audio samples into output analog audioaccording to said sampling clock.
 2. The audio/video I/O apparatus ofclaim 1 further comprising:a general purpose computer with a storagedevice and a peripheral slot; and peripheral interface circuitryinstalled in said peripheral slot and configured to transfer saiddigital video and digital audio samples from said audio input and videoinput ports to said storage device, and to transfer data from saidstorage device to said audio output and video output ports.
 3. Theaudio/video I/O apparatus of claim 2 wherein said peripheral interfacecircuitry further comprises FIFOs for coupling asynchronous operation ofsaid computer to synchronous operation of said video and audio input andoutput ports.
 4. The audio/video I/O apparatus of claim 2 wherein saidperipheral interface circuitry further comprises a CODEC for compressingsaid digital video into compressed video data for storing on saidstorage device and for decompressing said video data as they aretransferred from said storage device to said video output port.
 5. Theaudio/video I/O apparatus of claim 1 wherein the video input portcircuit further comprises:a detector configured to detect asynchronizing signal in said input analog video; a frequency generatorfor generating said video-rate clock and clocking said video outputport; a pulse generator configured to provide an input synchronizationpulse to said audio I/O port circuit, said input synchronization pulsebeing generated for each frame of said input analog video and beingbased on said detected synchronizing signal; and a pulse generatorconfigured to provide a synchronizing pulse to said audio I/O portcircuit.
 6. The audio/video I/O apparatus of claim 1 wherein said videoI/O port circuit further comprises a frequency generator configured togenerate said video-rate clock independent of both input and outputvideo.
 7. The audio/video I/O apparatus of claim 1 wherein saidfrequency synthesizer comprises an m/n phase lock loop to generate saidaudio sampling clock as an adjustable ratio of said video-rate clock. 8.The audio/video I/O apparatus of claim 7 wherein said m/n phase lockloop includes an m counter, an n counter, a phase detector, and anoscillator.
 9. The audio/video I/O apparatus of claim 8 furthercomprising a digital signal processor programmed to:selectivelyassociate a varying number of digital audio samples to each respectiveframe of said digital video to achieve a precise ratio of the number ofsaid digital audio samples over a finite time to the number ofassociated frames of said digital video over said finite time; andselectively associate a varying number of said digital audio samples toeach respective frame of said digital video as said digital audiosamples and said digital video are presented to said video and audiooutput ports.
 10. The audio/video I/O apparatus of claim 1 wherein saidfrequency synthesizer further comprises:an oversampled clock synthesizerthat generates an oversampled clock whose frequency is at least twice asgreat as the frequency of said audio sampling clock; and a frequencydivider that frequency divides said oversampled clock to generate saidaudio sampling clock.
 11. The audio/video I/O apparatus of claim 1further comprising a digital signal processor programmed to:selectivelyassociate a varying number of digital audio samples to each respectiveframe of said digital video to achieve a precise ratio of the number ofsaid digital audio samples over a finite time to the number ofassociated frames of said digital video over said finite time; andselectively associate a varying number of said digital audio samples toeach respective frame of said digital video as said digital audiosamples and said digital video are presented to said video and audiooutput ports.
 12. The audio/video I/O apparatus of claim 1 wherein saidaudio input port further comprises:an analog-to-digital converter forconverting said input analog audio into said digital audio samples, saidanalog-to-digital converter being clocked by said audio sampling clock;and a digital-to-analog converter for converting said digital audiosamples to said output analog audio, said digital-to-analog converterbeing clocked by said audio sampling clock.
 13. The audio/video I/Oapparatus of claim 1 further comprising:a host computer with a storagedevice for storing said digital audio samples and said digital video;first and second FIFOs for storing said digital audio samples betweensaid audio input port and said storage device; third and fourth FIFOsfor storing said digital audio samples between said storage device andsaid audio output port; and a digital signal processor for controllingflow of said digital audio samples between said first and second FIFOsand said third and fourth FIFOs.
 14. An audio/video input apparatus foracquiring one or multiple channels of digital audio samples, theapparatus comprising:a video input port configured to convert inputanalog video into digital video and generate a video-rate clock; afrequency synthesizer connected to receive said video-rate clock fromsaid video input-port and configured derive an audio sampling clockbased on said video-rate clock; and an audio input port configured tosample and convert input analog audio into digital audio samplesaccording to said sampling clock.
 15. The audio/video input apparatus ofclaim 14 further comprising:a general purpose computer with a storagedevice and a peripheral slot; and peripheral interface circuitryinstalled in said peripheral slot and configured to transfer saiddigital video and digital audio samples from said audio input and videoinput ports to said storage device.
 16. The audio/video input apparatusof claim 15 wherein said peripheral interface circuitry furthercomprises a FIFO for coupling asynchronous operation of said computer tosynchronous operation of said video and audio input ports.
 17. Theaudio/video input apparatus of claim 15 wherein said peripheralinterface circuitry further comprises a CODEC for compressing saiddigital video into compressed video data for storing on said storagedevice.
 18. The audio/video input apparatus of claim 14 wherein thevideo input port further comprises:a detector configured to detect asynchronizing signal of said input analog video; and a pulse generatorconfigured to provide an input synchronization pulse to said audio inputport, said input synchronization pulse being generated for each frame ofsaid input analog video and being based on said detected synchronizingsignal.
 19. The audio/video input apparatus of claim 14 wherein saidvideo input port further comprises a frequency generator configured togenerate said video-rate clock independent of input analog video. 20.The audio/video input apparatus of claim 14 wherein said frequencysynthesizer comprises an m/n phase lock loop to generate said audiosampling clock as an adjustable ratio of said video-rate clock.
 21. Theaudio/video input apparatus of claim 20 wherein said m/n phase lock loopincludes an m counter, an n counter, a phase detector, and anoscillator.
 22. The audio/video input apparatus of claim 21 furthercomprising:a digital signal processor programmed to selectivelyassociate a varying number of digital audio samples to each respectiveframe of said digital video to achieve an accurate ratio of the numberof said digital audio samples over a finite time to the number ofassociated frames of said digital video over said finite time.
 23. Theaudio/video input apparatus of claim 14 wherein said frequencysynthesizer further comprises:an oversampled clock synthesizer thatgenerates an oversampled clock whose frequency is at least twice asgreat as the frequency of said audio sampling clock; and a frequencydivider that frequency divides said oversampled clock to generate saidaudio sampling clock.
 24. The audio/video input apparatus of claim 14further comprising:a digital signal processor programmed to selectivelyassociate a varying number of digital audio samples to each respectiveframe of said digital video to achieve an accurate ratio of the numberof said digital audio samples over a finite time to the number ofassociated frames of said digital video over said finite time.
 25. Theaudio/video input apparatus of claim 14 whereinsaid audio input portfurther comprises an analog-to-digital converter for converting saidinput analog audio into said digital audio samples, saidanalog-to-digital converter being clocked by said audio sampling clock.26. The audio/video input apparatus of claim 14 further comprising:ahost computer with a storage device for storing said digital audiosamples and said digital video; first and second FIFOs for storing saiddigital audio samples between said audio input port and said storagedevice; and a digital signal processor for controlling flow of saiddigital audio samples between said first and second FIFOs.
 27. Anaudio/video output apparatus for synthesizing one or multiple channelsof digital audio samples into output audio, the apparatus comprising:avideo output port configured to synthesize output analog video fromdigital video and generating a video-rate clock; a frequency synthesizerconnected to receive said video-rate clock from said video output portand configured to derive an audio sampling clock based on saidvideo-rate clock; and an audio output port configured to synthesizeoutput analog audio from digital audio samples according to saidsampling clock.
 28. The audio/video output apparatus of claim 27 furthercomprising:a general purpose computer with a storage device and aperipheral slot; and peripheral interface circuitry installed in saidperipheral slot and configured to transfer data from said storage deviceto said audio output and video output ports.
 29. The audio/video outputapparatus of claim 28 wherein said peripheral interface circuitryfurther comprises a FIFO for coupling asynchronous operation of saidcomputer to synchronous operation of said video and audio output ports.30. The audio/video output apparatus of claim 28 wherein said videoperipheral further comprises a CODEC for decompressing said video dataas they are transferred from said storage device to said video outputport.
 31. The audio/video output apparatus of claim 27 wherein saidvideo output port further comprises:a frequency generator for generatingsaid video-rate clock and clocking said video output port; and a pulsegenerator configured to provide a synchronizing pulse to said audiooutput port.
 32. The audio/video output apparatus of claim 27 whereinsaid video output port further comprises a frequency generatorconfigured to generate said video-rate clock independent of outputvideo.
 33. The audio/video output port apparatus of claim 27 whereinsaid frequency synthesizer comprises an m/n phase lock loop to generatesaid audio sampling clock as an adjustable ratio of said video-rateclock.
 34. The audio/video output apparatus of claim 33 wherein said m/nphase lock loop includes an m counter, an n counter, a phase detector,and an oscillator.
 35. The audio/video output apparatus of claim 27wherein said frequency synthesizer further comprises:an oversampledclock synthesizer that generates an oversampled clock whose frequency isat least twice as great as the frequency of said audio sampling clock;and a frequency divider that frequency divides said oversampled clock togenerate said audio sampling clock.
 36. The audio/video output apparatusof claim 27 further comprising:a digital signal processor programmed toselectively associate a varying number of said digital audio samples toeach respective frame of said digital video as said digital audiosamples and said digital video are presented to said video and audiooutput ports.
 37. The audio/video output apparatus of claim 27whereinsaid audio output port further comprises a digital-to-analogconverter for converting said digital audio samples to said outputanalog audio, said digital-to-analog converter being clocked by saidaudio sampling clock.
 38. The audio/video output apparatus of claim 27further comprising:a host computer with a storage device for storingsaid digital audio samples and said digital video; first and secondFIFOs for storing said digital audio samples between said storage deviceand said audio output port; and a digital signal processor forcontrolling flow of said digital audio samples between said first andsecond FIFOs.
 39. A method for acquiring video and one or more channelsof audio and storing them in digital form on a storage device of acomputer, the method comprising the steps:receiving said video at avideo input port; detecting one or more synchronization signals in saidvideo; frequency-synthesizing an audio sampling clock from saidsynchroniztion signals in a frequency synthesizing circuit connected toreceive said synchronization signals from said video input port;receiving said audio at an audio input port, said audio input portclocked by said audio sampling clock; and storing said received videoand received audio on the storage device.